Free radicals, or reactive oxygen species, are generated in vivo by a variety of mechanisms including oxidative stress, redox cycling, and ionizing radiation. For example, hydrogen peroxide (H.sub.2 O.sub.2) can be generated endogenously by a number of cellular processes. H.sub.2 O.sub.2, once in the nucleus of a cell, can lead to formation of hydroxyl radicals which then cause site-specific damage to DNA such as oxidation of DNA bases. Depending on the mechanism which generates the free radicals, free-radical induced oxidative DNA base modifications include 8-hydroxy-guanine, thymine glycol, formamido remnant, dihydrothymine, 5-hydroxymethl uracil, and 5-hydroxy-5-methylhydantoin.
Oxidative DNA damage has been associated with several pathological conditions including Alzheimer's disease (Mecocci et al., 1994, Ann. Neurolog. 36:747-751; Prashad et al., 1996, Proc. Natl. Acad. Sci. USA 93:5146-50), radiation exposure (Wilson et al., 1992, Cancer Res. 48:2156-2162), ischemic damage and stroke (1996, NIH Guide 25), metal toxicity (Carmichael et al., 1995, Mutat. Research 326:235-43), breast cancer (Djuric et al., 1996, Cancer 77:691-6), carcinogenesis (Ames et al., 1995, Proc. Natl. Acad. Sci. USA 92:5258-65), and other inflammatory processes. For example, the process of carcinogenesis is believed to require. multiple genetic and/or epigenetic events which affect patterns of expression, or result in mutational alteration, of genes. Molecular mechanisms, including oxidative DNA damage and alteration of a cell's ability to repair damaged DNA, may lead to the development of genomic instability. Genomic instability is believed to occur in an early step in the process of carcinogenesis.
There is a need for sensitive assays for specific lesions resulting from oxidative DNA damage. For example, it is known to those skilled in the art that exposure of an individual to UV irradiation, ionizing radiation, or certain chemo-therapeutic agents, can result in oxidative DNA damage. Thus, there is a need for means, such as the methods and compositions according to the present invention, which are clinically useful to assess exposure to chemotherapy or radiation therapy. Additionally, for individuals at high risk of a pathological condition associated with oxidative DNA damage, it may be useful to monitor the levels of specific DNA base modifications resulting from oxidative DNA damage prior to clinical onset of the condition. However, with the exception of thymine glycol which can be measured using an immunoassay (Hubbard-Smith et al., 1992, Radiat. Res. 130:160-5; U.S. Pat. No. 5,552,285) or by mass spectrometric analyses (Markey et al., 1993, Ann. NY Acad. Sci. 679:352-7), quantitation of each specific DNA base modification has been hindered by the lack of sensitivity of existing assays to detect biologically significant levels of each modification amongst the variety of base modifications that may be present.